1. Field of the Invention
The present invention relates to surface hardening of steel workpieces. In particular, the present invention is a method of hardening selected surface areas of steel cutting instruments, such as cutting rules or knife blades, using laser beams to perform both surface hardening and stress relief of the workpiece.
2. Description of the Related Art
Typically, hardening of metals has been performed by carburizing, induction heating and, more recently, laser heat-treating. In conventional gas carburizing methods, a steel workpiece is heated in an atmosphere of a selected gas. Materials from the gas dissolve in the surface of the workpart becoming part of the crystalline structure. For example, a steel workpart is heated in an atmosphere of CO.sub.2 causing minute amounts of carbon to be liberated on the surface of the hot metal and to dissolve in the metal. A subsequent heat treatment to form a martensitic microstructure on the surface produces a hard surface. A martensitic microstructure is formed by heating the steel above the critical temperature--the temperature at which the steel changes phases from a ferrite or cementite microstructure to an austenite microstructure--and rapidly cooling, or quenching, the steel to form a new microstructure phase, martensite. Martensite is the hardest of the steel microstructure phases.
However, the rapid cooling required to produce martensite also induces internal stresses within the microstructure that make the martensite brittle. Therefore, a subsequent tempering process is required to relieve these internal stresses. Tempering typically entails heating the steel to a temperature below the critical temperature for several hours. Heating the steel below the critical temperature avoids inducing a microstructure phase change back to austenite, but also reduces some of the hardness of the martensite. The hardness reduction is the result of some of the carbon particles trapped in the martensite being released. Thus, the microstructure before tempering appears as untempered martensite and after tempering the microstructure appears as tempered martensite.
Some drawbacks are present in surface hardening by carburizing. One such drawback is that it is difficult to surface harden only selected areas of the workpart. In order to only harden selected areas, those surfaces not to be hardened must be masked. The masking prevents those surfaces from being subjected to the gas atmosphere, thereby preventing hardening of the masked surface. The masking process is often difficult, time-consuming and unreliable due to the intense heat of the carburizing process. Another drawback of carburizing is controlling the depth of the hardened surface. Carburizing typically requires post-processing machining, such as grinding, in order to obtain the desired hardened case depth. Carburizing also requires an additional tempering process after the quenching process in order to stress relieve the part. Such a stress relief process typically entails placing the workpiece in an oven, often for a period of several hours. This significantly increases both the cost and the amount of time to process the workpiece.
Another known method of surface hardening steel workparts is induction heating. In induction heating, the steel workpart is placed within an induction coil. An electrical current is passed through the induction coil which induces secondary currents to flow along the surface of the workpart. The secondary current flow causes the surface of the workpart to be preferentially heated. As the electrical current in the induction coil is increased, the surface of the workpart is heated above the critical temperature, thus causing a microstructure phase change to austenite. When the workpart is rapidly cooled, or quenched, a martensitic microstructure is formed. Thus, when only a shallow surface of the part is heated above the critical temperature and is rapidly quenched, only the shallow surface is transformed into a martensitic microstructure while the remainder of the part remains unchanged. This shallow surface of martensite forms the hard surface.
However, the rapid cooling induces internal stresses that cause the steel part to become brittle. Therefore, a subsequent tempering process is required to relieve the internal stresses.
Induction heating has some of the same drawbacks as carburizing. Namely, it is difficult to harden only selected surface areas and the steel workpart requires a post-hardening tempering process that is costly and time-consuming.
Additionally, shallow hardened case-depths are difficult to achieve with induction hardening. Typically, the case depth is controlled during induction hardening by producing a higher frequency current in the induction coil. However, common induction heating machines present limitations on the highest frequency available. Common induction heating machines have a frequency limit of about 1 MHz. However, if a case depth of 0.004-0.006 was desired, an induction machine frequency of approximately 10 MHz would be required. Such a machine is costly and commonly only available in Europe.
Induction heating has been the most common method of producing steel cutting rules. Steel cutting rules produced by induction heating generally provide good bendability properties, thereby allowing the rules to be formed into a number of shapes. However, induction heated rules generally have low durability properties, thereby requiring frequent replacement. Additionally, induction heated steel cutting rules require air or liquid quenching during the heat-treating process which causes thin rules to warp and further requires tempering to relieve internal stresses. The tempering process typically lowers the surface hardness previously obtained during the heat treating step. Therefore, common induction hardened rules are typically hardened to only about 55 R.sub.c.
Another known method of surface hardening is laser heat-treatment. Various types of lasers are available for heat treating workpieces, including continuous wave CO.sub.2 lasers. Laser heat treatment using a CO.sub.2 laser typically entails applying an absorbent substance, such as black oxide or phosphate coatings, to the surface area of the part to be heated. This coating reduces reflection of the laser beam and focuses the energy of the laser beam to the area to be hardened. The laser beam is then focused, via a lens or the like, which generates an intense energy flux that rapidly heats the surface.
One distinct advantage of laser heat treatment is that the laser beam may be controlled to heat the surface of the metal piece above the critical temperature to a depth of only a few thousandths of an inch or less. Controlling the depth of the heating to this shallow level allows for self quenching. That is, no liquid or air quenching is required. Self-quenching is accomplished by conduction due to the mass and temperature disparity between the portion of the workpart not heated by the beam and the small depth of the surface heated above the critical temperature by the beam. The heat on the surface is quickly transferred to the unheated portion thereby quenching the heated surface. However, the self-quenching process has been taught to be undesirable for thin parts such as knife blades and therefore air or liquid quenching has been particularly advisable. Air or liquid quenching is required due to the insufficient mass of the part to facilitate the conduction. The addition of such air or liquid quenching increases both the cost and the processing time.
One such method of laser-treating steel workparts is disclosed in U.S. Pat. No. 4,304,978. This patent teaches laser heat treating a flat part, such as a knife or blade, by focusing a laser beam perpendicular to the major flat surface of the part using a cylindrical lens. The width of the beam is adjusted according to the desired width of the part to be heated. The part is then moved through the laser or the laser may be moved along the part to heat the surface. U.S. Pat. No. 4,304,978 teaches that thin parts, such as a knife blade, requires gas quenching to prevent melting of the part. Therefore, one shortcoming of U.S. Pat. No. 4,304,978 is that the laser treated part, such as a knife blade, is not self quenching.
Therefore, it is desirable to provide a method of hardening a steel cutting rule or knife blade so as to obtain equivalent or superior ductility properties as common induction heated rules, but with superior wear resistance. It is also desirable that the method provide for self quenching of the cutting rule or knife blade to reduce processing time and cost.
Further, it is desirable to provide a method of stress relieving the heat treated cutting rule that reduces the processing time and cost without weakening the metal part.